This invention relates to gas discharge devices and, more particularly, to gas discharge devices which utilize a cathode having a micro hollow array.
The general concept of a discharge device which utilizes a hollow cathode for increased current capability is disclosed in the prior art. A hollow cathode glow discharge utilizing a single, nearly spherical hollow cathode is described by A. D. White in Journal of Applied Physics, Vol. 30, No. 1, May 1959, pp. 711-719. The author reported a stable discharge and negligible deterioration from sputtering. The basic mechanisms contributing to the hollow cathode effect are described by G. Schaefer et al. in Physics and Applications of Pseudosparks, Ed. by M. A. Gundersen and G. Schaefer, Plenum Press, New York, 1990, pp. 55-76. Measurements of the temporal development of hollow cathode discharges are described by M. T. Ngo et al. in IEEE Transactions-on Plasma Science, Vol. 18, No. 3, Jun. 1990, pp. 669-676.
A variety of hollow cathode structures for fluorescent lamps have been disclosed-in the prior art. A directly-heated hollow cathode having an interior coating of an emissive material is disclosed in U.S. Pat. No. 4,523,125, issued Jun. 11, 1985 to Anderson. A shielded hollow cathode for fluorescent lamps is disclosed in U.S. Pat. No. 4,461,970, issued Jul. 24, 1984 to Anderson. A hollow electrode having an interior coating of an emissive material that varies in thickness is disclosed in U.S. Pat. No. 2,847,605, issued Aug. 12, 1958 to Byer. A short arc fluorescent lamp having hollow cathode assemblies is disclosed in U.S. Pat. No. 4,093,893, issued Jun. 6, 1978 to Anderson. Cup shaped electrodes containing emissive material for use in cold cathode fluorescent lamps are disclosed in U.S. Pat. No. 3,906,271, issued Sep. 16, 1975 to Aptt, Jr., and U.S. Pat. No. 3,969,279, issued Jul. 13, 1976 to Kern. A fluorescent lamp wherein a filament is positioned within a cylindrical shield is disclosed in U.S. Pat. No. 2,549,355, issued Apr. 17, 1951 to Winninghoff. Additional hollow cathode discharge devices are disclosed in U.S. Pat. No. 1,842,215, issued Jan. 19, 1932 to Thomas; U.S. Pat. No. 3,515,932, issued Jun. 2, 1970 to King; U.S. Pat. No. 4,795,942, issued Jan. 3, 1989 to Yamasaki; U.S. Pat. No. 3,390,297, issued Jun. 25, 1968 to Vollmer; and U.S. Pat. No. 3,383,541, issued May 14, 1968 to Ferreira.
An electrical discharge electrode including a plurality of tubes, which are filled with an electron emissive material and embedded in a metallic matrix, is disclosed in U.S. Pat. No. 4,553,063, issued Nov. 12, 1985 to Geibig et al.
A variety of.different fluorescent lamp types have been developed to meet different market demands. In addition to conventional tubular fluorescent lamps for office and commercial applications, compact fluorescent lamps have been developed as incandescent lamp replacements. Subminiature fluorescent lamps have found applications in displays and general illumination in limited spaces.
Different fluorescent lamps may operate under very different discharge conditions. The small size of subminiature fluorescent lamps may not allow for hot cathode operation, thus requiring efficient cold cathode emitters. The buffer gas pressure in subminiature fluorescent lamps is often on the order of 100 torr in order to limit electron loss to the lamp wall. By contrast, conventional fluorescent lamps typically utilize pressures on the order of 0.5-2.0 torr. Environmental concerns have necessitated the investigation of lamp fill materials other than mercury. In mercury-free fluorescent lamps, radiation is often produced by excimers of inert gases, such as neon, argon and xenon. In order to form excimers, a gas pressure of approximately 100 torr is required. In subminiature fluorescent lamps utilizing cold cathodes, the operating life may be limited by sputtering. In addition, current limitations may restrict light output. These trends have produced a need for improved cathode configurations.
The hollow cathode configurations disclosed in the prior art are not suitable for use in subminiature fluorescent lamps because of their relatively large sizes and because of the relatively high pressures utilized in subminiature fluorescent lamps.
Hollow cathodes have been studied in connection with other applications, such as excitation sources for gags lasers, ion sources, plasma jets, electron beams and plasma switches. In each case, a cathode with a single, relatively large opening, or hollow, has been studied at low (subtorr) pressure.
According to the invention, a discharge device for operation in a gas at a prescribed pressure comprises a cathode and an anode spaced from the cathode, and electrical means for coupling electrical energy to the cathode and the anode. The cathode comprises a conductor having a plurality of micro hollows therein. Each of the micro hollows has cross-sectional dimensions selected to support a micro hollow discharge at the prescribed pressure. Electrical energy is coupled to the cathode and the anode at a voltage and current for producing micro hollow discharges in each of the micro hollows in the cathode.
Each of the micro hollows preferably has a cross-sectional dimension that is on the order of the mean free path of electrons in the gas. Under these conditions, electrons undergo oscillatory motion within the micro hollows and produce substantially higher currents than a planar cathode. The micro hollow discharges operate independently of each other.
The prescribed pressure for operation of the discharge device is preferably in a range of about 0.1 torr to atmospheric pressure. The discharge device may include a discharge chamber for maintaining the prescribed pressure. When the discharge device is operated at or near atmospheric pressure in air, the discharge chamber may not be required.
The discharge device may include a dielectric layer between the cathode and the anode. The dielectric layer is preferably disposed of a surface of the cathode and is provided with openings aligned with the micro hollows. The dielectric layer is preferably utilized when the spacing between the cathode and the anode is greater than about the mean free path of electrons in the gas. The dielectric layer ensures that a glow discharge between the cathode and the anode terminates in the micro hollows.
According to a first application of the discharge device, a fluorescent lamp comprises a sealed, light-transmissive tube containing a gas at a prescribed pressure, and first and second spaced-apart electrodes mounted within the tube. The first electrode comprises a conductor having a plurality of micro hollows therein. Each of the micro hollows has dimensions selected to support a micro hollow discharge at the prescribed pressure. The fluorescent lamp further includes electrical means for coupling electrical energy to the first and second electrodes at a voltage and current for producing micro hollow discharges in each of the micro hollows in the first electrode. The fluorescent lamp preferably includes a phosphor coating on the inside surface of the light-transmissive tube. The phosphor coating emits radiation having a prescribed spectrum in response to radiation generated by the discharge between the first and second electrodes. Each of the micro hollows preferably has a cross-sectional dimension that is on the order of the mean free path of electrons in the gas.
For AC operation of the fluorescent lamp, the second electrode preferably comprises a conductor having a plurality of micro hollows therein. Each of the micro hollows in the second electrode has dimensions selected to produce a micro hollow discharge at the prescribed pressure.
The fluorescent lamp preferably includes a dielectric layer on the surface of each electrode. Each dielectric layer has openings aligned with the micro hollows.
In a second application of the discharge device, a radiation source comprises a sealed discharge tube containing a gas at a prescribed pressure, first and, second spaced-apart electrodes mounted within the discharge tube, add electrical means for coupling electrical energy to the first and second electrodes. At least one of the electrodes comprises a conductor having a plurality of micro hollows. Each of the micro hollows has dimensions selected to produce a micro hollow discharge at the prescribed pressure. In a preferred embodiment, the radiation source is an excimer lamp wherein the gas and the prescribed pressure are selected to emit radiation in a wavelength range of about 80 to 200 nanometers.
In a third application of the discharge device, a laser for generating laser radiation at a predetermined wavelength comprises a first mirror that is substantially reflective at the predetermined wavelength, a second mirror that is partially reflective and partially transmissive at the predetermined wavelength, a chamber for enclosing a gas at a prescribed pressure between the first and second mirrors, and a laser pumping device positioned between the first and second mirrors. The laser pumping device comprises a cathode having at least one micro hollow therein, the micro hollow having dimensions selected to produce a micro hollow discharge at the prescribed pressure, an anode spaced from the cathode and electrical means for coupling electrical energy to the cathode and the anode at a voltage and current for producing the micro hollow discharge in the micro hollow. The laser pumping device provides an unobstructed optical path along the optical axis between the first and second mirrors. The cathode may include a plurality of micro hollows and the anode may include a plurality of openings aligned with the micro hollows. In this case, each of the micro hollows defines an optical axis between the first and second mirrors for a generation of multiple laser beams at the predetermined wavelength. Two or more of the laser pumping devices may be disposed along the optical axis between the first and second mirrors.
In a fourth application of the discharge device, a light source comprises a sealed discharge chamber containing a gas at a prescribed pressure, a cathode mounted within the discharge chamber and an anode spaced from the cathode. The cathode comprises a conductor that defines an array of micro hollows. Each of the micro hollows has a cross-sectional dimension selected to support a micro hollow discharge at the prescribed pressure and has an axial dimension that is substantially less than the cross-sectional dimension. The light source further comprises electrical means for coupling electrical energy to the cathode and the anode at a voltage and current for producing micro hollow discharges in each of the micro hollows in the cathode. The light source is preferably configured as a thin, flat light source.
The light source may be.configured as a flat fluorescent light source, including a phosphor coating on a light-transmissive portion of the discharge chamber. The phosphor coating emits radiation having a prescribed spectrum in response to radiation generated within the micro hollows.
In a preferred embodiment, the cathode of the flat light source comprises a wire mesh including spaced-apart conductors which define the micro hollows. Alternatively, the cathode may comprise a conductive pattern formed on a light-transmissive substrate, the conductive pattern comprising a grid of spaced-apart conductive lines.
In an additional application, the discharge device of the present invention can be configured as an electron source for generating multiple electron beams. In a further application, the discharge device is configured as an ion source for generating multiple ion beams.